When using a compass to plan or plot a vehicle route on a flat map, there is a difference between the path covered by the points of the compass and the path travelled by the aircraft because the aircraft path follows a curve. This difference is sometimes referred to as the "chord defect". In an effort to compensate for this defect, a known hand device uses a non-circular pair of gear wheels. The movable point of the compass in this device rotates about a pivot point which is displaced relative to the axis of the compass leg carrying the other compass point, introducing an additional defect which cannot be compensated for in a simple fashion by the spiral, curve-like tooth arrangements of the two gears. Furthermore, the production of such gears is very expensive. This arrangement is shown in German OS No. 2146881.
In another prior art device, the chord defect is compensated by arranging the lengths of both legs of the compass so that the lengths are longer by a predetermined factor than the radius which mathematically corresponds in the case of certain open angles to the length of the circular arc pertaining to those angles. However, one cannot achieve compensation for the chord defect acting uniformly over the entire possible angular area as a result of this lengthening. This approach is shown in German Utility Pat. No. 7927078.